On the Cisco 2900 I’ve done the following: Created 3 vlans: 10, 20
& 30, assigned 5 ports to each vlan, set fa0/20 as the trunk port
using 802.1q and assigned an IP to each vlan so I can ping test once
complete.

On the Juniper I’ve: Created 3 sub interfaces eth1.1, 1.2 & 1.3,
specified eth1 to use 802.1q, assigned an ip to each interface in the
same subnet as the corresponding vlans on the Cisco, specified the same
vlan tags as on the Cisco 2900, Made all 3 sub-interfaces members of the
trust zone and I can’t get any traffic across the trunk. I’ve even
resorted to changing the cable